Tintin's struggling with Haddock's sixth sense (8)
I believe the answer is:
instinct
'sense' is the definition.
(instinct can mean an in-built sense or intuition)
'tintin's struggling with haddock's sixth' is the wordplay.
'struggling with' indicates anagramming the letters.
'haddock's sixth' becomes 'c' (6th letter of 'haddock').
'tintins' anagrammed with 'c' gives 'INSTINCT'.
(Other definitions for instinct that I've seen before include "Natural impulse governing behaviour" , "Gut reaction, impulse" , "Inborn impulse to behave in certain way" , "Innate behavioural pattern" , "Innate feeling" .)